The present invention relates to an air-fuel ratio feedback control method of an internal combustion engine, and more specifically to an air-fuel ratio feedback control method using an electrical digital computer.
An internal combustion engine, in general, emits gases containing pollutants such as carbon monoxide (CO), nitrogen oxides (NOx), unburned or partly burned hydrocarbons (HC). When these pollutants are to be cleaned using a three-way catalytic converter, it is required to very precisely control the air-fuel ratio within a range around the stoichiometric air-fuel ratio such that all of the three components, i.e., CO, NOx and HC can be removed effectively.
Therefore, the internal combustion engine employing the above-mentioned three-way catalytic converter usually adopts a method of controlling the feedback of air-fuel ratio responsive to signals from a concentration sensor (exhaust gas sensor) which detects the concentrations of particulr components in the exhaust gas. Among many concentration sensors, an oxygen concentration sensor (Hereinafter referred to as O.sub.2 sensor) for detecting the oxygen concentration has been extensively used for automobiles, such as a stabilized zirconia element or a titania element. When the air-fuel ratio in the atmosphere hovers around 14.5 (stoichiometric air-fuel ratio), the O.sub.2 sensor of this type exhibits suddenly changed electric properties. In other words, the O.sub.2 sensor detects the changes in the air-fuel ratio causing the electric signals thereof to change.
The O.sub.2 sensors, however, have different characteristics depending upon the individual units, and they also exhibit a great variation in temperature characteristics. Therefore, in order to control the air-fuel ratio over a wide range of temperatures of the engine, while suppressing control errors that may stem from individual characteristics, a particular contrivance must be provided to treat the output voltage of the O.sub.2 sensor. One method may be to vary or control a reference voltage for comparison. Namely, the output voltage of the O.sub.2 sensor is compared with the reference voltage by a comparator, to discriminate whether the air-fuel ratio at the present moment is rich or lean. In this case, the reference voltage for comparison can have variable values responsive to a maximum value in the output voltage of the O.sub.2 sensor.
The conventional air-fuel ratio control systems of this type, which rely upon variable values for comparison, employ an analog control circuit. Accordingly, only very simple control functions could be obtained from the complicated circuitry, and it was very difficult to accomplish optimum control of the air-fuel ratio maintaining a high precision. As mentioned earlier, in order to effectively clean the exhaust gas by using a three-way catalytic converter, it is necessary that the air-fuel ratio is precisely controlled to an optimum value responsive to the operation condition. With conventional analog control systems, it is almost impossible to attain an optimum air-fuel ratio control without using complex circuitry, which causes the costs to be increased.